1. Nature 374, 663 (1995).
2. Progress towards the Eradication of Poliomyelitis, Status Report March 1994, WHO/GPV/POLIO/94.1 (WHO, Geneva).
3. Lancet 345, 786 (1995).
4. Nathanson, N., Langmuir A.D. Am. J. Hyg. 78, 16–81 (1963).
5. EPI Newsletter 16 (6), 6, (1994).